The ventral tegmental area (VTA) is a core component of the neural circuitry that drives goal-directed behavior and a primary target upon which drugs of abuse manipulate behavior. Although generally regarded as a dopaminergic nucleus, about half of VTA neurons signal through release of the amino acid neurotransmitters GABA or glutamate. Unlike dopamine, these transmitters activate fast ionotropic postsynaptic receptors indicating VTA neurons signal across multiple modes and timescales. Recent evidence suggests important roles for VTA GABA neurons in encoding negative reward value, but little is known regarding the behavioral relevance of the recently identified glutamate neurons in the VTA. However, developmental, anatomical, and electrophysiological evidence suggests overlapping but distinct roles for VTA glutamate and dopamine signaling in the neural circuit processes underlying behavioral reinforcement. The goal of this proposal is to identify how excitatory VTA signaling contributes to the processes underlying behavioral reinforcement and how VTA glutamate neurons functionally integrate into the mesolimbic neural circuits that regulate motivated behavior. We will use mouse genetics, optogenetics, and viral vectors to selectively manipulate discrete excitatory VTA circuits. Behavioral, anatomical, electrophysiological, and molecular assays will then be used to assess VTA glutamate neurons, determine their physiological and behavioral functions, and define the anatomical and molecular factors distinguishing them from neighboring dopamine and GABA-releasing neurons. Together these studies comprise a comprehensive plan to define the form and function of a novel class of VTA neurons while enhancing our understanding of intrinsic VTA heterogeneity. These studies will identify new opportunities and specify new targets for the treatment and prevention of drug addiction and other compulsive behavioral disorders.